Understanding Joules and  Milliwatts – Photopolymer Plate Exposure

Is there a right or wrong?

Our Technical Solutions Group (TSG) always references mW (milliwatts) when referencing lamp intensity, but the product brochures of some plate manufacturers reference exposure times (sec) and joules (mJ/cm2). If you look further down on the brochures you will see these values are based on an assumed lamp intensity, for example, 16 mW/cm2. How many joules per second are in 1 milliwatt?

The answer short answer is: 1 mW = 0.0010 J/s; in other words, the joules per second unit number 0.0010 J/s converts to 1 mW, one milliwatt. 1 W = 1 Joule per second.

Here’s another good question, “Why do manufacturers reference joules and not just time and mW/cm2?”

TSG Answer: The plate manufacturer does not know what the mW/cm2 of every exposure unit will be so they have to offer a unit of measure that allows for input of the varying intensity of the user’s lamps, i.e. mW/cm2. The “joules” that plate manufacturers reference is a measurement of energy it takes to adequately cross-link the monomers (polymerize) the photopolymer material to the desired main or back exposure. Although this can be confusing because it brings lamp intensity and time into the equation, mJ/cm2 is a great number to know because it allows the technician to increase or decrease plate exposure time (sec) based on the measured lamp intensity.

Example

1. The MacDermid LUX ITP-60 Product Brochures states that to back expose a .067” plate with a targeted relief of .020”, it requires 672 mJ/cm2 and should expose for 42 seconds based on a lamp intensity of 16 mW/cm2.
   1. 16 mW/cm2 x 42 seconds = 672 mJ/cm2  [16 x 42 = 672].
   2. Therefore if your lamps measured 20 mW/cm2 you would divide your known desired value of 672 mJ/cm2 by 20mW/cm2 and you would get the exposure time required.
   3. 672 / 20 = 33.6 (seconds). Because the lamps were stronger than assumed the back exposure time dropped by 8.4 seconds.   
2. The MacDermid LUX ITP-60 Product Brochures then states that to main expose a .067” plate, it requires 13.8 J/cm2 and should expose for 10 min (600 seconds) based on a lamp intensity of 23 mW/cm2
   1. First of all please note that the unit of measure changed from mJ/cm2 to J/cm2. This means the sum will be divided by 1000 to convert mJ to J.
   2. 23 mW/cm2 x 600 seconds (10 min) = 13,800 mJ/cm2  [23 x 600 = 13,800]. Note: 13,800 mJ = 13.8 J
   3. Therefore if your lamps measured only 15 mW/cm2 you would divide your known desired value of 13,800 mJ/cm2 (13.8 J/cm2)  by 15mW/cm2 and you would get the exposure time required.
   4. 13,800 / 15 = 920 (seconds). Because the lamps were weaker than assumed the main exposure time increased by 320 seconds. 

For you history buffs

• The watt is named after the Scottish inventor James Watt. This unit was proposed initially by C. William Siemens in August 1882 in his President's Address to the Fifty-Second Congress of the British Association for the Advancement of Science.
• The term joule(s) is the energy dissipated as heat when an electric current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule (1818–1889) 

If you want to learn more about this subject, please feel free to reach out to us by calling 1-800-445-4017, or by e-mail at [email protected].

All Printing Resources has formed our Technical Solutions Group (TSG) to encompass our full range of expertise in all critical areas of the flexo process. This team is made up of industry professionals dedicated to being up-to-date on new technologies along with best practices. They are armed with the latest in diagnostic tools and are experienced in problem-solving that can achieve sustainable results. The TSG has walked in your shoes, and have felt your pain.